[GHC] #8582: Record syntax for pattern synonyms

[GHC]

#8582: Record syntax for pattern synonyms
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        Reporter:  cactus            |                Owner:  mpickering
            Type:  feature request   |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:  5144              |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1152
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Comment (by mpickering):

 Simon, I didn't want to write an explicit specification for this patch
 because it would amount to copying the specification for records.
 Which bit did you find confusing in the example I gave? In retrospect, the
 phrase "relevant data constructor" is confusing. So to explain, by
 "relevant" I mean an isomorphic (normally defined) data constructor. Does
 the example not make this any clearer?

 The problem with the implementation is with the record updates. Pattern
 synonym builders have required constraints which normal data constructors
 don't have. When the record update is desugared, it is necessary to
 provide the dictionaries for these constraints to the builder. My question
 was, how was I meant to do this. I ended up adding a field to the
 RecordUpd constructor which carried around the HsWrapper which then
 applied the dictionaries. More details are to be found in the last comment
 on the phab ticket.

 I don't have much time anymore to work on this ticket but I would be very
 disappointed it if did make it into GHC 8.0 as I started working on it
 several months ago.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8582#comment:15>
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